Recently, there is an increasing demand toward the increase in the capacity and speed of radio communication. Studies are vigorous as to the methods for improving the effective utilization ratio of definite frequency resources, as one method of which attentions are drawn to the technique to make use of space domains. The space-domain utilization techniques includes, as one, an adaptive array antenna (adaptive antenna) wherein, by adjusting the amplitude and phase by means of a weighting coefficient for multiplication on a received signal (hereinafter, referred to as “weight”), reception is intense for the signals arriving in a desired direction, thus enabling suppression in directions of interference waves. This can improve the communication capacity for the system.
Meanwhile, there are other arts utilizing space domains, i.e. 1) space division multiple access technique for transmission to different mobile stations (hereinafter, referred to as “SDMA” wherein SDMA is an abbreviation of space division multiple access) and 2) space division multiplex technique for transmission to the same mobile station (hereinafter, referred to as “SDM” wherein SDM is an abbreviation of space division multiplex), of different data sequences by use of the physical channels same in time, frequency and sign through utilization of a spatial orthogonality over the propagation path. The SDMA technique is disclosed of information in JP-A-2002-261670 and in Document T. Ohgane et al, “A study on a channel allocation scheme with an adaptive array in SDMA,” (IEEE 47th VTC, Page(s): 725-729 vol. 2 1997). Where the spatial correlation coefficient between mobile stations is lower than a predetermined value, SDMA is available thus making it possible to improve the throughput of a radio communication system and the number of simultaneous active users.
Meanwhile, the SDM technique is disclosed of information in JP-T-2001-505723 and in Document G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” (Bell Labs Tech. J, pp. 41-59, Autumn 1996), wherein the transmitter and the receiver both has a plurality of antenna elements thus realizing SDM transmission under the propagation environment low in received signal correlation between the antennas. In this case, different data sequences are sent from a plurality of antennas provided on the transmitter by use of physical channels same in time, frequency and sign on an antenna-element-by-antenna-element basis. At the receiver, demultiplex-reception is made based on different data sequence from the received signal at a plurality of antennas provided on the receiver. This allows to achieve speed increase by use of a plurality of space division multiplex channels instead of using multi-level modulation. In implementing SDM transmission, communication capacity can be increased in proportion to the number of antennas on condition that the transmitter and the receiver have the equal number of antennas, in an environment that a multiplicity of scatterers exist between the transmitter and the receiver under satisfactory S/N (signal-to-noise ratio) conditions.
However, in the conventional SDM art, the maximum number of space division multiplex ones undergoes restriction at the end of transmitter and receiver which is less in the number of antennas. Consequently, where there is a deviation in the number of transmission and receive antennas, space division multiplex is possibly not utilized efficiently under certain propagation environments. Particularly, because antenna elements can be set up greater in the number at the base station end than at the mobile station, there arises a case to cause a room for the degree of spatial freedom in transmission of from the base station to the mobile station. Meanwhile, in order to make the mobile station compatible with SDM, there is a need for a plurality of antennas, a plurality of transmission or reception systems and a signal processing section for demultiplexing a space-division-multiplexed signal, thus raising cost. For this reason, it can be considered that the mobile stations not compatible with SDM coexist within the communication area, thus requiring a method for space division multiple access under coexistence of mobile stations compatible with space division multiplex and mobile stations uncompatible therewith. Meanwhile, SDMA, when to implement, usually employs space division based on directive beams. In case SDM is done furthermore, beam-to-beam spatial correlation is increased resulting in a propagation condition not suited for SDM in general cases.